In recent years, as function and performance of portable electronic devices, such as a notebook computer (hereinafter, referred to as a “notebook PC”) and a mobile phone, are improved, power consumption of the portable electronic devices is increasing. Accordingly, the capacity or energy density of a cell of a secondary battery used for the portable electronic device also tends to further increase. With such an electronic device, in order to obtain a voltage suitable for an operation of the portable electronic device and an electric capacity sufficient for supplying power for a long period of time, a battery used for the portable electronic device has often taken the form of a battery pack having a plurality of battery cells accommodated in a housing by connecting the battery cells to one another in combination of series and parallel connections.
Hitherto, a secondary battery using a water-soluble electrolyte, such as a nickel cadmium (Ni—Cd) battery or a nickel hydrogen (NiMH) battery, has been adopted as a battery cell, a lithium ion (Li-ion) battery excellent in the mass energy density (Wh/kg) and the volume energy density (Wh/l) has been recently adopted as a battery cell. In the lithium ion battery, cobalt acid lithium is used for a positive electrode, a carbon material is used for a negative electrode, and an organic electrolyte obtained by dissolving lithium salt in a combustible organic solvent is used as an electrolyte. The organic electrolyte is used because the organic electrolyte has such a characteristic that lithium easily reacts with water. Accordingly, a case or a metal jacket is employed for forming a cell of a lithium ion battery and has a sealed structure.
In the secondary battery using the water-soluble electrolyte, even if cell voltage and temperature increase at a late stage of electric charging thereby resolving the electrolyte and generating oxygen gas, the electrolyte returns to original water due to the oxygen cycle or a catalyst plug. However, an electrolyte of the lithium ion battery does not return to the original state once the electrolyte is resolved. Accordingly, if a charging voltage or a charging current increases up to a predetermined value, or more, or the lithium ion battery is used under a condition of any abnormality to thereby increase the temperature, the electrolyte is resolved and gas is generated. As a result, an increase in the internal pressure occurs. For this reason, a gas discharge valve is provided in the case of a cell of the lithium ion battery in order to prevent explosion.
With regard to the form of a battery pack, there has been provided the form of hard pack in which a battery cell is accommodated in a hard plastic case and the form of soft pack in which a battery cell is packed with a heat-shrinkable tube. In case where a plurality of cells is accommodated, the hard pack is adopted because it is easy to handle the hard pack. In the hard pack in which a plurality of battery cells, each consisting of a lithium ion battery is accommodated, a protection circuit is generally provided, which monitors a charging current, a charging voltage, the temperature of a cell and the like, and cuts off a circuit as required. In a recent battery pack, a so-called intelligent battery system is adopted for realizing such a protection circuit and for accurately measuring the residual capacity, which system per se is comprised of a controller or various kinds of sensors incorporated in the battery pack for measuring and controlling a state of charge and discharge, and for further notifying the system of the state of an electronic device.
In many cases, a battery pack mounted in a notebook PC constitutes a part of a housing of the body of the notebook PC. In addition, in order to increase a space where battery cells are accommodated, a so-called extension-type battery pack having a part of a housing protruding from a housing of the notebook PC is adopted. On one hand, when a user uses a notebook PC inside a company, it is common to carry the notebook PC between a conference room and each office without putting the notebook PC in a case or a bag. Accordingly, the likelihood is that the notebook PC might be accidentally dropped onto the floor. At this time, a strong impact to which the weight of the notebook PC is added is loaded on a battery pack, which may eventually cause short-circuiting or breakage of an internal circuit of a battery cell. As a result, the temperature of the cell comes to rise rapidly, and thus high-temperature combustible gas might be discharged from a gas discharge valve to the inside of the battery pack.
When the battery cell is exposed to high-temperature combustible gas for a predetermined time or more, the internal temperature goes up. As a result, the combustible gas is emitted from the gas discharge valve. The high-temperature combustible gas that has been emitted may catch fire due to an electrical circuit inside the battery pack or may cause spontaneous combustion due to high temperature. Since a plurality of battery cells is densely accommodated in a battery pack, there is also a risk such that another battery cell or other battery cells will be heated by the high-temperature combustible gas and combustible gas will be discharged from the heated battery cells, and the temperature inside the housing will be extremely increased, and as a result, all battery cells accommodated in the battery pack will be burnt down by a spreading fire. In addition, the amount of heat energy of the discharged gas increases as the battery cells are closer to a fully charged state, which increases a risk of fire spreading.
Japanese Unexamined Patent Publication (Kokai) No. 8-293327 teaches a battery pack capable of preventing occurrence of fire breakout which might be caused by an evaporated gas of an electrolyte containing an combustible organic solvent by providing a partition wall between a battery compartment which contains battery cells, and an electrical circuit compartment which contains an electrical circuit.
Japanese Unexamined Patent Publication (Kokai) No. 2003-331803 teaches a technique of separating batteries from one another in a battery pack, which contains a plurality of batteries, in order to suppress transmission of heat to a neighboring battery when heat is generated in a certain battery. The same document also teaches a double structure in which a partition wall is formed of a synthetic resin and a space is interposed for defining a duplicated structure to thereby improve a heat insulating effect.
Japanese Unexamined Patent Publication (Kokai) No. 2003-151525 teaches a battery charging and discharging device in which a plurality of batteries are accommodated in a manner such that the batteries are separated from one another by respective separating plates. The separating plates are formed by using fire-resistant polycarbonate, fire-resistant PET, or ABS resin, for example.
An influence of heat generation among battery cells has been suppressed by using a partition wall formed of a fire-resistant synthetic resin or by providing a space inside, as described in the foregoing related art. However, in recent years, as the energy density of a battery cell increases and the number of battery cells integrally packaged in a battery pack increases, it cannot be said that the fire-resistant partition wall described in the related art above is sufficient for suppressing the influence of heat generation among battery cells. For example, in a lithium ion battery used in a notebook PC, there is a case where approximately 1 through 2 liters of combustible gas having a temperature of about 600° C. through 700° C. is discharged from each battery cell. The partition wall formed of the fire-resistant synthetic resin cannot withstand a rise in the temperature caused by high-temperature combustible gas discharged from a battery cell but melts away within a short time. This may cause gas to be discharged from the neighboring battery cells, and as a result, there is such a likelihood that all battery cells might be burnt down. Particularly, in the case of an extension-type battery pack, it is requested to further improve safety because a large mechanical impact may be provided on a battery cell when, for example, the battery pack is dropped. Accordingly, in addition to a protection circuit of a conventionally existing intelligent battery, a novel system capable of fully securing the safety against an impact of a battery pack is required.
Therefore, a need exists to provide a battery pack improved in safety thereof. Further, a need exists to provide a battery pack which is improved in safety thereof against drop and/or impact. Furthermore, a need exists to provide a portable electronic device, which is able to mount therein such a battery pack.